1. Field of the Invention
This invention relates to the removal of finely divided particles from liquids and more particularly to an electrostatic apparatus for separating finely divided particles from nonconducting liquids.
2. Description of the Prior Art
Many oils contain excessive amounts of finely divided suspended solid particles that lower the quality of the oils or interfere with their further processing. The particles may be suspended in oils such as crude oils as those oils are produced from wells or may be introduced into the oils during processing. For example, particles of iron sulfide or iron oxide may be introduced into hydrocarbon oils in certain processing steps in petroleum refineries. If the oils are to be further processed by passing them through fixed beds of catalyst at elevated temperatures and pressures, the solid particles may deposit on the catalyst, particularly at the inlet end of the catalyst beds and eventually plug the catalyst beds. In the hydrogenation of vegetable oils, finely divided particles of catalyst are suspended in the oil during hydrogenation and must be removed from the hydrogenated oil to give a product of acceptable clarity.
Conventional filtering of liquids in which the liquid is made to pass through a permeable medium or screens is effective in removing from the liquid solid particles having a nominal diameter larger than about 25 microns. The solid particles of iron oxide and iron sulfide that become suspended in petroleum fractions in some petroleum refining operations are less than 5 microns in diameter and the majority of the particles are less than one micron. Mechanical-type filters such as papers, cloths, screens or stacked discs are not effective in removing such particles from the liquid. If the openings in the filter medium are reduced in size to prevent passage of the solid particles, the filter medium quickly becomes plugged.
In U.S. Pat. No. 3,324,026 of Waterman et al. and U.S. Pat. No. 3,394,067 of Shirley, electrofilters are described which include a solid urethane foam filter element between the electrodes to aid in separation of the solids from the liquid being filtered. Such foams cannot be used in the filtration of hot refinery streams. Moreover, backflushing of the separated solids from the filter element is not possible; consequently, it is necessary to replace the filter element each time it becomes loaded with sufficient solids to cause excessive current through the filter.
U.S. Pat. No. 2,534,907 of Ham et al and U.S. Pat. No. 2,573,967 of Hamlin describe electrostatic filters in which the liquid to be filtered is made to pass between electrodes in a bed of a filter medium. Ham et al use a particulate bed of an organic ion-exchange resin as the filter medium. Hamlin prefers a loosely packed maze of fibrous material such as glass wool as a filter medium but suggests that a granular non-conductive material can be used. Neither of the patents describes an electrostatic filter or precipitator in which the particulate bed can be cleaned effectively by backflushing to recondition the bed for another filtration cycle. Hamlin suggests partially cleaning the filter medium by disconnecting the source of electric potential and circulating a liquid through the filter bed in the same direction as during the filtration operation.
U.S. Pat. No. 3,928,158 of Fritsche et al. describes an electrostatic filter in which an electrostatic gradient is imposed across a bed that consists of substantially spherical, nondeformable, smooth-surfaced beads of a ceramic material of high electrical resistivity. Glass beads are preferred. A liquid is caused to flow through interelectrode space within the bed while the electrical gradient is imposed across the bed. Solid particles are deposited in the filter bed principally at the points of contact of the substantially spherical particles. The filter bed can be repeatedly and readily reconditioned for further use by backflushing to remove the deposited solids. U.S. Pat. Nos. 3,799,855; 3,799,856 and 3,799,857 of Franse describe different applications of a filter of the type described in U.S. Pat. No. 3,928,158 of Fritsche et al.
U.S. Pat. No. 3,252,885 of Griswold and U.S. Pat. No. 2,925,372 of Keehn describe electrostatic filters in which the liquid being filtered flows radially through the filter medium. In the filters described in both of those patents, the radial flow is used to increase the area of the filter medium on which deposition of solids can occur. Neither of the patents discloses a filtering structure which can be cleaned for further use by backflushing.